Rodney's right. I'm not sure if what you mean is cold-peel transfers, like you'd see in the 70's.
But what I think your asking about is "clear-gel" specialty ink. More or less, it's a colorless gel that gets printed the *same* way as any other plastisol ink. You can print it by itself or print it over other colors. Pretty common in screenprinting circles. Look up some of the ink supply companies for more info.
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